WO2024008311A1

WO2024008311A1 - Bidirectional active filter

Info

Publication number: WO2024008311A1
Application number: PCT/EP2022/069122
Authority: WO
Inventors: Bernhard Wunsch; Stanislav Skibin; Ville FORSSTROM; Yannick Maret
Original assignee: Abb Schweiz Ag
Priority date: 2022-07-08
Filing date: 2022-07-08
Publication date: 2024-01-11

Abstract

An active filter (16) for reducing electromagnetic noise comprises an injection impedance (Zinj) and a controlled current source (24), wherein the injection impedance (Zinj) and the controlled current source (24) are connected in series between an electric line (18) and ground (22) and wherein the controlled current source (24) comprises a operational amplifier (36), which output current depends on a current or voltage of the electric line (18).

Description

DESCRIPTION

Bidirectional active filter

FIELD OF THE INVENTION

The invention relates to an active filter for reducing electromagnetic noise, in particular for power applications.

BACKGROUND OF THE INVENTION

High frequency active filters, either analog or digital, are an attractive alternative to purely passive filters and enable size and/or cost reduction of an EMC (electromagnetic compatibility) filter. Passive EMC filters due to their large inductors add significant cost and size to converter systems and cross talk can strongly downgrade their performance.

In high-power applications, however, the large operational currents and/or voltages exceed by far the specifications of typical components used in an active filter. The dominant frequencies of these large currents and voltages are typically well below the EMC frequency range of interest. Furthermore, a large low-frequency content and a DC offset, which may arise from the use of non-ideal electronic components, may lead to saturation of the active components of an active filter.

DESCRIPTION OF THE INVENTION

Active filters may comprise controlled sources and may be classified by the quantity that is being sensed and the quantity that is controlled. Both sensing and output quantity may be either current or voltage which results in four possible combinations, i.e. current-controlled current-source, current-controlled voltage-source, voltage-controlled current-source and voltage-controlled voltage-source.

In active filters based on voltage actuation, the injection mechanism is usually realized by an inductor on the power line between source and grid (or more general victim), which usually also requires a significant number of turns. For large power converters, the power lines are realized by thick cables or large busbars so that the inductors are bulky and costly. Therefore, an active filter based on current actuation is typically preferred for high-power application, where the controlled source is placed on a shunt connection between noise source (such as a converter) and the noise victim (such as a load or a grid).

An active filter based on voltage-sensing current actuation behaves as a controlled shunt impedance. This topology can effectively filter noise from the converter, if the controlled shunt impedance is smaller than the converter output impedance and the victim impedance. There may be no need of inductors for sensing or actuation inductor. However, the low shunt impedance may be ineffective and in fact even harmful for a voltage noise source on the victim/grid side. Such a noise voltage on the grid side may be caused by inductive coupling.

For a voltage noise source on the converter side, a controlled voltage source in the shunt may be beneficial since the introduced shunt impedance offers an additional filtering of the noise. In contrast, for a voltage noise source on the grid side, the finite shunt impedance of a controlled voltage source increases the noise current at the grid side. In contrast for a controlled current source, the shunt impedance is large (ideally infinite) and therefore does not introduce an additional propagation path for grid side noise. For a bidirectional filter, the implementation with controlled current sources may be beneficial, but it comes with the challenge to shape the low frequency behavior and DC bias behavior.

A problem for active filters with current injection may be a significant amount of low frequency (LF) current and voltage below the desired frequency range, e.g., 150 kHz to 30 MHz. The compensating current generated by the active filter may have to be injected into the circuit through an injection capacitor, which has a high impedance at low frequency for any relevant capacitance values. Low frequency components may strongly contribute to the active filter output voltage range. The output voltage required to compensate low frequency current harmonics may exceed voltage capabilities of currently available electronic components used in the active filter, such as operational amplifiers and/or transistors. Another problem may be that the low frequency noise from the main circuit couples through the injection capacitor to the output of the active filter, creating low frequency ripple voltage. The relevant output active filter voltage must come on top of those low frequency ripples.

A second problem for active filters with current injection may be nonidealities of the amplifier circuit, which may cause an additional DC-current component at the output of the current source. Because of the injection method through a high impedance/capacitor, this DC current may also cause a large DC voltage of the amplifier and can lead to saturation. The two problems described above may lead to a compliance limitation of the active filter current source. The effects significantly increase the required voltage span to achieve the wanted compensation. Low-frequency components below the frequency range of interest may therefore have to be limited at the output of the active filter, regardless of their origin. Additionally, both effects may cause saturation of the components of the active filter, since the required large bandwidth components may have a limited voltage span of their power supply.

It is an objective of the invention to overcome above described problems. Further objectives of the invention are to provide an active filter for high power applications, which is small in size, economic and has a good performance.

These objectives are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

The invention relates to an active filter for reducing electromagnetic noise. The active filter may be a bidirectional filter interconnected between a noise source, such as a converter and a noise victim, such as a load or grid. The active filter may be connected to a power line between the converter and the load or grid. Electromagnetic noise may be higher order harmonic components and/or ripple in the voltage in the power line. The term “power” may refer to voltages larger than 100 V and/or currents of more than 1 A.

According to an embodiment of the invention, the active filter comprises a injection impedance and a controlled current source. The injection impedance and the controlled current source are connected in series between an electric line and ground or more general a reference point provided by a constant voltage source. The controlled current source comprises a operational amplifier, which output current depends on a current or voltage of the electric line. The injection impedance may be provided by an injection impedance circuit and/or may comprise a capacitor and optionally a resistor and/or inductor.

The differential input may be provided between an inverting and a non-inverting input and/or may be proportional to a current and/or voltage in the electric line. The controlled current source and/or the operational amplifier may be supplied by a constant voltage source. The controlled current source may be a Howland current source.

The electric line may be a power line between a converter and a grid or load.

The topology of the active filter uses a controlled current source and filters noise on either side of the active circuit. In addition to the noise source stemming from the switching action of a converter, the active filter also may filter noise from a grid or load, which may be induced by parasitic cross talk coupling. In many converter topologies, there is a strong input-output coupling introducing a grid side noise voltage. In such a way, the active filter enables filtering of noise sources located on either side of the active filter. Additionally, the active filter reduces size and cost of an EMC filter. For example, the weight of magnetic components of the active filter may be reduced by more than 70% compared to a passive filter.

According to an embodiment of the invention, the injection impedance is connected between the electric line and the controlled current source. Furthermore, the injection impedance may comprise a capacitor. This may decouple a DC component of the voltage in the electric line from the controlled current source. The injection impedance additionally may comprise an inductor and/or a resistor, which may be connected in series with the capacitor. This may improve the stability of the active filter for higher frequencies, such as a frequency range of interest, for example an EMC frequency range and/or above 150 kHz.

According to an embodiment of the invention, an output of the controlled current source is connected to the injection impedance. An output current of the current source dampens and/or equalizes noise, i.e. high frequency components, in the electric line.

According to an embodiment of the invention, a sense impedance is connected between an output of the operational amplifier and an output of the controlled current source. The sense impedance may be provided by a sense impedance circuit. The sense impedance is used to sense an output current of the operational amplifier and/or an amplified current thereof. The sense impedance is used to control the current gain. The sense impedance may be shaped that gain of the output current of the controlled current source is higher in a frequency range of interest, for example an EMC frequency range and/or above 150 kHz, and smaller in other frequency ranges, in particular lower frequencies.

According to an embodiment of the invention, the output of the controlled current source is connected to an input of the operational amplifier. The operational amplifier controls the output current such that the input voltage balances the voltage drop caused by the output current flowing through the sense impedance.

According to an embodiment of the invention, the sense impedance comprises a resistor connected in series with a capacitor and/or the sense impedance comprises a resistor connected in parallel with a capacitor. In particular, the output current Io is controlled such that the input voltage Vi balances the voltage drop caused by the output current flowing through the sense impedance Zs multiplied with a gain factor k, leading to Vi=k x Zs x Io. Both the gain factor k and the sense impedance may be designed to be constant and may be realized with resistors. When the sense impedance is realized by a resistor in series with a parallel connection of a resistor with a capacitor, this increases the sense impedance at low frequencies.

According to an embodiment of the invention, an input impedance is connected into a control input of the controlled current source. The input impedance of the controlled current source is used to control the current gain. The control input may be provided with a voltage indicative of a current and/or voltage in the electric line. The input impedance may comprise a first input impedance connected to a non-inverting input and/or a second input impedance is connected to an inverting input of controlled current source and/or the operational amplifier.

According to an embodiment of the invention, the input impedance and/or the first input impedance and/or the second input impedance comprises a resistor and capacitor connected in series. The input impedance may be provided by an input impedance circuit. The input impedance may shape a frequency dependence of the gain factor k and/or may be designed to be large at low frequencies, such as frequencies below the EMC range of interest, which may start at 150 kHz.

A frequency dependent gain factor, input impedance and/or sense impedance results in a stabilization of the DC-operation point of the topology and a shaping of the current source frequency response.

Nonidealities of the active circuit comprising the operational amplifier, like input bias currents and offset voltage, may lead to a DC bias current at the output of the controlled current source. A DC current may cause a large load voltage leading to saturation of the controlled current source. The DC output current can be strongly reduced by increasing the sense impedance and/or the gain factor k at low frequency.

When the sense impedance and/or the gain factor decrease with increasing frequencies, the current gain of the controlled current source has the shape of a high pass filter. This may limit a low-frequency current injection and therefore may avoid saturation of the controlled current source.

The sense impedance and the gain factor as described above may stabilize the DC- operation point of active filter and at the same time may reduce a low-frequency spectrum of the generated output current without compromising the active filter performance in the filtering frequency range.

According to an embodiment of the invention, a ground impedance is connected between an output of the controlled current source and a reference point, such as ground. The ground impedance may be seen as an output sink and/or may be provided by a ground impedance circuit. The reference point may be a reference point for the supply voltage of the controlled current source. For example, the reference point may be a midpoint in a constant voltage source.

The ground impedance may be frequency dependent and/or frequency selective and/or may protect the controlled current source, and in particular the active circuit comprising the operational amplifier, against large operational currents and/or voltages.

According to an embodiment of the invention, the ground impedance comprises an inductor and in particular an inductor and resistor connected in series.

The ground impedance may reduce a low-frequency output voltage of the active circuit. Together with the injection impedance, which may be an injection capacitor, the ground circuit may create a voltage divider and may reduce the output voltage of the active filter for the frequency range, where the ground impedance is smaller than the injection impedance. If the ground impedance is realized as inductor or as an inductor and resistor connected in series, it may have the following effects.

The performance of the active filter is mostly unaffected by the ground impedance and its bidirectionality is preserved. At the frequency range of interest, the impedance of the ground impedance may be chosen larger than that of the injection impedance.

The low frequency output voltage of the active filter is limited. The ground impedance may have a low impedance at low frequency: any low frequency current generated by the active filter can flow toward the reference point, i.e. ground, without requiring a large low- frequency voltage at the output of the active filter.

The low frequency ripple voltage is reduced. In the absence of the ground impedance, the low frequency output impedance of the active filter would be comparable or higher than that of the injection impedance. Common-mode low frequency voltage noise between the electric line and ground mostly falls over the active filter, causing low frequency voltage ripple at the output of the active filter. The ground impedance decreases the low frequency output impedance of the controlled current source and ensures that most of the low frequency ripple voltage falls over the injection impedance instead. The ground impedance ensures a DC path to ground or more general to the reference point. In case of the usage of a Howland current source-based active filter, a ground impedance in the form of a capacitor prevents a DC-current from the active filter to the reference point. A missing DC current path from the Howland current source may downgrade the performance, since the output voltage at DC is undefined and can take any value.

In summary, the ground impedance reduces the active filter output voltage at low frequency, regardless of its origin.

According to an embodiment of the invention, the controlled current source comprises a further current amplifier connected between an output of the operational amplifier and an output of the controlled current source. This further amplifier may allow to compensate noise and ripple in electric lines with even higher current rating. The operational amplifier and the further current amplifier may form an active circuit of the active filter.

According to an embodiment of the invention, the further current amplifier comprises two transistors connected in series in a constant current source, with inputs of the transistors connected to the output of the operational amplifier and with a midpoint between the transistors connected to the output of the controlled current source. The further current amplifier may be a class AB amplifier, i.e. an amplifier designed for amplifying both positive and negative parts of the currents.

According to an embodiment of the invention, the further amplifier is connected between the operational amplifier and the sense impedance. In such a way, also nonidealities of the further amplifier are considered by the sense impedance.

According to an embodiment of the invention, the non-inverting input of the controlled current source is connected via a first resistive feedback impedance and via a sense impedance with an output of the controlled current source.

According to an embodiment of the invention, the inverting input of the controlled current source is connected via a second resistive feedback impedance with the output of operational amplifier and/or the output of the active circuit.

The first resistive feedback impedance and/or the second resistive feedback impedance may comprise or may be a resistor. The first resistive feedback impedance and/or the second resistive feedback impedance may be provided by a first resistive feedback impedance circuit and/or the second resistive feedback circuit. According to an embodiment of the invention, a sensing voltage between the noninverting input and the inverting input of the controlled current source is proportional to a current in the electric line. This may be achieved with a transformer for current sensing.

According to an embodiment of the invention, the active filter further comprises a transformer with a primary winding interconnected in the electric line and a secondary winding interconnected between a non-inverting input and an inverting input of the controlled current source.

According to an embodiment of the invention, a resistor is connected in parallel to the secondary winding.

The input impedance may be interconnected between the transformer and the input of the controlled current source. The first input impedance may be connected to the non-inverting input, in particular between the transformer and the operational amplifier. The second input impedance may be connected to the inverting input, in particular between the transformer and the operational amplifier. The first input impedance and/or the second input impedance may be interconnected into a current-sensing path.

According to an embodiment of the invention, the electric line is connected between an electric grid and a converter. The active filter is connected to the electric line in a connection point between the electric grid and the converter. The electric line may be provided by an output of a converter supplied by the electric grid.

According to an embodiment of the invention, the current or voltage is sensed between the electric grid and the connection point or the current or voltage is sensed between the converter and the connection point. The transformer may be provided between the electrical grid and the connection point or the transformer may be provided between the converter and the connection point. The active filter may be a feed backward active filter or feed forward active filter depending on the relative location of the sensing part (such as the transformer) and actuation part (i.e. the controlled current source). If the sensing occurs closer to the noise source than the actuation, then the active filter may be of feed forward type and if the sensing occurs closer to the noise victim than the actuation, then the active filter is of backward type.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

The subject-mater of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows a n active filter according to an embodiment of the invention.

Fig. 2 schematically shows an active filter according to a further embodiment of the invention.

Fig. 3 schematically shows an active filter according to a further embodiment of the invention.

Fig. 4 schematically shows an active filter according to a further embodiment of the invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 and 2 show a system 10 comprising a noise victim 12, a noise source 14 and an active filter 16 interconnected between the noise victim 12 and the noise source 14. The noise victim 12 may be an electric grid. The noise source 14 may be an electrical converter. The noise victim 12 and the noise source 14 are connected by an electrical line 18, to which the active filter 16 is connected to at a connection point 20. At the other end, the active filter 16 is grounded, or more general connected to a reference point 22, to which also the noise victim 12 and the noise source 14 are connected.

The active filter 16 comprises a injection impedance Z_inj and a controlled current source 24, which are series connected between the connection point 20 and the reference point 22. The injection impedance Z_inj is connected between the electric line 18 and the controlled current source 24. The output current of the controlled current source 24 depends on a current or voltage of the electric line 18.

The system 10 shown in Fig. 1 and 2 is represented by a single-phase equivalent, but equally applies to a multiphase system like the common mode current propagation of a three- phase system. On the right hand side of Fig. 1, additionally an equivalent circuit 26 of the system 10 is shown. The objective of the active filter 16 is to suppress the high frequency components of the current I_LISN in the electric line 18. In addition to the noise source U_src, for example from a converter, the system 10 comprises a noise source U_ind, for example on the grid side, which may be caused from cross talk like magnetic couplings. For ideal current source behavior with infinite output impedance, a simple circuit analysis of Fig. 1 leads to

where Z_LISN is the impedance of the noise victim 12, Z_src is the impedance of the noise source 14 and Z_in is the impedance of the noise source 14 together with the active filter 16.

Here, “feed backward” refers to the topology shown in Fig. 1, wherein the current or voltage, which is supplied to the controlled current source 24, is sensed between the electric grid and/or noise victim 12 and the connection point 20. “feed forward” refers to the topology shown in Fig. 2, wherein the current or voltage, which is supplied to the controlled current source 24, is sensed between the electric converter and/or noise source 14 and the connection point 20.

Both topologies are applicable for bidirectional filter usage because they allow to equally suppress grid side and source side noise. For good filter performance, a high gain |F| » 1 is required for the feed backward configuration whereas an accurate gain of F → 1 (zero phase and magnitude 1) is required for the feed forward configuration.

Fig. 3 shows a further embodiment of an active filter 16, with a ground impedance or ground impedance Z_gnd connected in parallel to the controlled current source 24. The embodiment of Fig. 3 can be combined with the embodiments of Fig. 1 and 2, i.e. the current or voltage measurement also may be performed between the noise source 14 and the connection point 20.

The ground impedance Z_gnd is realized as R-L or L branch and forms a voltage divider of low frequency-noise and a low impedance path for the injected current I_inj, thereby reducing the output voltage U_AEF, of the filter circuit 16.

For an ideal controlled current source 24 and without adding an external ground impedance, Z_gnd → ∞ and the circuit analysis results in: ⁽¹⁾

As derived in the above equation, the active filter consists in a multiplication of the source impedance with the current gain leading to the term Z_src(1 + F). Without active filter, Z_tot ⁼ Z_LISN + Zsrc the condition for a good filter performance of the active filter is

For typical realizations of the active filter, the controlled-current source 24 is realized by a combination of high frequency transistors and/or an operational amplifier. The output voltage V_AEF and output current I_AEF are then limited according to the specifications of these components. It turns out that in many applications, the voltage U_AEF across the active filter 16 has a large low frequency content, below the frequency range where the active filter 16 needs to filter. One reason can be the rather large injection impedance Z_inj which is typically capacitive, while the impedance Z_LISN is inductive at low frequency, while Z_src depends on the topology of the converter. Low frequency noise can stem from low harmonics of the grid frequency or the switching frequency. The impact of Z_inj can be eliminated when the gain F is strongly reduced at low frequency, which for F → 0 then results in U_AEF = but still the low frequency noise components mainly of

U_src can lead to prohibitively large voltages.

The equations (1) are modified by a finite ground impedance Z_gnd to:

Comparing the equations (1) and (2), one notices three major modifications when adding the ground impedance Z_gnd:

1) The voltage U_AEF is proportional to It is therefore possible to reduce the low

frequency components of U_AEF by choosing for low frequencies Z_gnd « Z_inj.

2) For a good filter performance of noise caused by U_src , one needs Z_tot > Z_LISN +

Z_src which can be either obtained by

min The latter condition corresponds to filtering by a shunt impedance

(Z_gnd + Z_inj), which improves for small shunt impedances.

3) The noise source U_ind on the victim side cannot be effectively filtered by a small shunt impedance (Z_gnd + Z_inj), in fact the filter performance decreases if

The required parameter regime to enable low U_AEF and good filtering at the same time is therefore:

• At low frequency: in order to reduce U_AEF

• At HF for good filter

performance

In practice, since Z_inj is typically a capacitor, Z_gnd could be an inductor, a series or parallel connection of an inductor and a resistor in series or simply a resistor.

Fig. 4 shows an active filter 16, which is realized with a current-controlled current-source 24.

A current transformer 28 is connected into the electric line 18. The transformer 28 has a primary winding 30 interconnected in the electric line 18 and a secondary winding 32 interconnected between a non-inverting input 34+ and an inverting input 34- of the controlled current source 24. The transformer 28 may be connected in a feed forward or feed backward configuration, such as described with respect to Fig. 1 and Fig. 2.

The controlled current source 24 comprises an operational amplifier 36, which provides the inputs 34+, 34- and which is supplied by a constant voltage source 38. An output 40 of the operational amplifier 36 is connected to a further current amplifier 42, which is also supplied by the constant voltage source 38. The reference point 22 is a midpoint between two series-connected parts of the constant voltage source 38, which both may provide the same voltage. The operational amplifier 36 and the further current amplifier 42 may form an active circuit 43 of the active filter 16.

An output 44 of the further current amplifier 42 and/or the active circuit 43 is connected to a sense impedance Z_sense, which provides the output 46 of the controlled current source 24.

The injection impedance Z_inj is connected between the electric line 18 and the output 46. Such as shown, the injection impedance Z_inj may comprise a capacitor 48. Optionally, the injection impedance Z_inj comprises an inductor 45 and/or a resistor 47 connected in series with the capacitor 48.

The optional ground impedance Z_gnd is connected between the output 46 and the reference point 22 and/or ground. Such as shown, the ground impedance Z_gnd may comprise an inductor 49 and a resistor 51 connected in series.

As shown, the sense impedance Z_sense may comprise a resistor 50 connected in series with a capacitor 52 and a resistor 54 connected in parallel with the capacitor 52.

The output 44 of the active circuit 43 and/or the further amplifier 42 is connected via the sense impedance Z_sense and a first resistive feedback impedance Z_FB+ to the non-inverting input 34+ of the operational amplifier 36. The output 44 of the active circuit 43 and/or the further amplifier 42 is connected via a second resistive feedback impedance Z_FB with the inverting input 34- of the operational amplifier 36. Each of the first resistive feedback impedance Z_FB+ and the second resistive feedback impedance Z_FB_ comprises a resistor 56, 58.

The secondary winding 32 of the transformer 28 is connected to a first input impedance Z_in+ and a second input impedance Z_in_ . The first input impedance Z_in+ is connected to the non-inverting input 34+ of the controlled current source 24 and may comprise a resistor 60 and capacitor 62 connected in series. The second input impedance Z_in_ is connected to the inverting input 34- of the controlled current source 24 and may comprise a resistor 64 and capacitor 66 connected in series. Furthermore, a resistor 68 may be connected in parallel to the secondary winding 32. The sense impedance Z_sense, which is used to sense the output current I_AEF, and the input impedance Z_in+, Z_in_ both may have frequency sensitive components, such as capacitors 52, 62, 66, which influence the frequency behavior of the active filter 16. The active circuit 43 controls the output current I_AEF, such that the input voltage V_in balances the voltage drop caused by the output current I_AEF flowing through the sense impedance Z_sense multiplied with a dimensionless gain factor k, leading to

Since the sense impedance Z_sense is realized by a resistor 50 in series with a parallel connection of a capacitor 52 with a resistor 54, the sense impedance Z_sense increases at low frequencies. Similarly, the input impedance Z_in+, Z_in , which influences the gain factor k, is realized as a capacitor 62, 66 series-connected with a resistor 60, 64 and is designed to be large at low frequencies. This results in the frequency behavior of the active filter 16 as described above.

The following analysis is not limited to a Howland current source but applies similarly to other current sources based on current sensing via a sensing impedance Z_sense. The total current gain can be estimated to:

Here, R _CT is the ohmic load of the resistor 68 at the secondary winding 32 of the current transformer 28, N₂ the turn number of the secondary winding 32 and Y_HCS the transadmittance of the Howland current source given below in (3).

The load impedance of the current source 24 is dominated at low frequencies by the injection capacitor 48, which represents a high impedance at low frequencies. Therefore, it is important that there is no output current I_AEF from the current source 24 at low frequencies and in particular at DC. The transconductance of a Howland current source is given by: (3)

Therefore, the output current I_AEF can be suppressed by increasing Z_sense and Z_in at low frequency which is achieved by the capacitors 62, 66 and by the capacitor 52 and resistor 54 in the sense impedance. We note that the low frequency spectrum can also be suppressed by using a high-pass filter for the input voltage V_in, which may be part of the current transformer 28, however, such an approach cannot reduce the DC-bias current caused by the active circuit

43 discussed below.

Even with zero input voltage V_in, imperfections of the amplifiers 36, 42, like input offset voltage between the terminals of the operational amplifier 36 and bias currents entering the input ports, will lead to a DC current at the output of the current source 24. The suggested modifications of the sense impedance Z_sense and partly also the gain factor k = Z_sense/Z_FB suppress this DC current component.

For example, the difference I_os = I_B+ - I_B- between the bias currents I_B+, I_B_ at the noninverting and inverting input causes a DC-bias current of (4)

Since under normal operation conditions Z_FB » Z_sense, the small bias current l_os is amplified by up to 4-order of magnitude. Since also the load impedance of the current source is large due to the capacitor 48, the active circuit 43 has a large DC-voltage offset and can saturate. This can be avoided by increasing the sense impedance Z_sense at low frequencies and at DC. The input offset voltage V_os between the input terminals of the operational amplifier 36 also causes a DC bias current at the output of the current source, but since this contribution is proportional to V_Os/Z_sense, it is also suppressed by increasing the sense impedance Z_sense.

The further current amplifier 42 is designed as a class AB amplifier and comprises two transistors 70, 72 connected in series in the constant current source 38. A midpoint between the transistors 70, 72 provides the output 44 of the active circuit 43. Two diodes 74, 76 and two resistors 78, 80, which are connected in series, are connected in parallel to the two transistors 70, 72. An inner midpoint between the diodes 74, 76 is connected to the output 40 of the operational amplifier. Outer midpoints between the diode 74 or 76 and the resistor 78 or 80 are connected to the gates of the transistors 70, 72.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An active filter (16) for reducing electromagnetic noise, the active filter (16) comprising: an injection impedance (Z_inj); a controlled current source (24); wherein the injection impedance (Zinj) and the controlled current source (24) are connected in series between an electric line (18) and ground (22); wherein the controlled current source (24) comprises an operational amplifier (36), which output current depends on a current or voltage of the electric line (18).

2. The active filter (16) of claim 1, wherein the injection impedance (Zinj) is connected between the electric line (18) and the controlled current source (24); wherein the injection impedance (Z_inj) comprises a capacitor (48); wherein the injection impedance (Z_inj) comprises an inductor (45) and/or a resistor (47); wherein an output (46) of the controlled current source (24) is connected to the injection impedance (Z_inj).

3. The active filter (16) of claim 1 or 2, wherein a sense impedance (Z_sense) is connected between an output (40) of the operational amplifier (36) and an output (46) of the controlled current source (24); wherein the output (46) of the controlled current source (24) is connected to an input (34+) of the operational amplifier (36).

4. The active filter (16) of claim 3, wherein the sense impedance (Z_sense) comprises a resistor (50) connected in series with a capacitor (52); wherein the sense impedance (Z_sense) comprises a resistor (54) connected in parallel with the capacitor (52).

5. The active filter (16) of one of the preceding claims, wherein an input impedance (Z_in+, Z_in_) is connected into a control input (34+, 34-) of the controlled current source (24); wherein the input impedance (Z_in+, Z_in_) comprises a resistor (60, 64) and capacitor (62, 66) connected in series.

6. The active filter (16) of one of the preceding claims, wherein a ground impedance (Z_gnd) is connected between an output (46) of the controlled current source (24) and a reference point (22).

7. The active filter (16) of claim 6, wherein the ground impedance (Z_gnd) comprises an inductor (49) and a resistor (51) connected in series.

8. The active filter (16) of one of the preceding claims, wherein the controlled current source (24) comprises a further current amplifier (42) connected between an output (40) of the operational amplifier (36) and an output (44) of the controlled current source (24).

9. The active filter (16) of claim 8, wherein the further current amplifier (42) comprises two transistors (70, 72) connected in series in a constant current source (38), with inputs of the transistors (70, 72) connected to the output (40) of the operational amplifier (36) and with a midpoint between the transistors (70, 72) connected to the output (44) of the controlled current source (24).

10. The active filter (16) of claim 8 or 9, wherein the further current amplifier (42) is connected between the operational amplifier (36) and a sense impedance (Z_sense).

11. The active filter (16) of one of the preceding claims, wherein a non-inverting input (34+) of the controlled current source (24) is connected via a first resistive feedback impedance (Z_FB+) and via a sense impedance (Z_sense) with an output (46) of the controlled current source (24); wherein an inverting input (34-) of the controlled current source (24) is connected via a second resistive feedback impedance (Z_FB_) with the output (44) of the operational amplifier (36).

12. The active filter (16) of one of the preceding claims, wherein a sensing voltage between a non-inverting input and an inverting input of the controlled current source (24) is proportional to a current in the electric line (18).

13. The active filter (16) of one of the preceding claims, wherein the active filter (16) further comprises a transformer (28) with a primary winding (30) interconnected in the electric line (18) and a secondary winding (32) interconnected between a non-inverting input (34+) and an inverting input (34-) of the controlled current source (24).

14. The active filter (16) of claim 13, wherein a resistor is connected in parallel to the secondary winding.

15. The active filter (16) of one of the previous claims, wherein the electric line (18) is connected between an electric grid (12) and an electric converter (14) and the active filter (16) is connected to the electric line (18) in a connection point (20) between the electric grid (12) and the electric converter (14); wherein the current or voltage is sensed between the electric grid (12) and the connection point (20) or the current or voltage is sensed between the electric converter (14) and the connection point (20).